I see two general classes of errors. The underlying communication is just TCP/VISA primitives so, in many cases, the API will just throw standard connection errors (error 56). For specific Modbus protocol error codes (ILLEGAL DATA VALUE) should also be thrown as errors and look to have the value -389110 - ErrorCode.
Now what exactly you do with those errors is up to you.

I would say here you are just describing what an Actor does and nothing specific to OOP. An Actor can be an object but doesn't have to be from my experience. I personally think that this use of inheritance can be the most difficult to actually understand/maintain as well. If you've ever used an NI's Actor Framework where the inheritance reaches depths of 5+, trying to understand what the Actor does can be quite difficult (though someone might claim this is because they violated SRP).
Perhaps I'm missing some historical context though and should read some older papers.

Not a lawyer but I suspect it's a bit of a good faith kind of thing similar to Visual Studio Community Edition or Visual Studio Code. I also imagine the licensing cost might be cheaper than the potential of getting caught and NI wanting to do something about it.

The red dot just indicates that LabVIEW will be coercing the data for you so whether there is a loss of information depends on what the data is being coerced from and to. For instance, if a U8 is being coerced into a U16 there isn't any loss of data but if we instead coerce the U16 into a U8 we are losing the top 8 bits of data (depending on what we know about this data this may or may not be an issue for us). FXP conversion will likely lose information because LabVIEW tries to preserver the value rather than keeping the bit value and changing the interpretation of those bits.
If you attach the project you are working out of we should be able help explain what would happen for your particular case.

What c-series module are you using? In the project, if you right-click the module under the FPGA target, some have the option to change their I/O between calibrated and raw. In this case raw would probably be much easier to work with.

It looks like the morning sessions (as well as the keynotes) on Monday are limited to Alliance Partners but the afternoon technical sessions are open to everyone.
Source: https://forums.ni.com/t5/NIWeek-Discussions/2019-NI-Week-Alliance-Day-open-to-the-general-public/td-p/3883876

This may be a misunderstanding from my end but I thought if you did not include any license then your code defaults to "not really open source". I'm not a lawyer but choosealicense.com/no-permission/ is where I was reading that and I have used the site before to get the TL;DR for licensing.